1. Field of the Invention
The present invention relates to mobile telephones and more particularly to mobile telephones such as on-vehicle telephones for use in automobile telephone systems or the like, portable telephones, and on-vehicle portable telephones.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a prior art frequency synthesizer for a mobile telephone described, for example, in the Transactions of National Conference of Institute of Electronics and Communication Engineers of Japan, Communications Section, Vol. No. 447, 1986, pp. 2-265. Referring to the figure, reference numeral 1 denotes a power source which is connected to a switch 2a for switching the power source 1 by receiving an intermittent signal output through intermittent control portion 60. 53 denotes a phase comparator for comparing phases of output signals from frequency dividers 58 and 59, and outputting an error signal to a charge pump 54. 55 denotes a loop filter through which the charge pump signal is transmitted to a voltage controlled oscillator (VCO) 56, 57 denotes a prescaler, 58 denotes a variable frequency divider for dividing an output signal of the prescaler 57, 59 denotes a reference frequency divider for generating a reference frequency, 60 denotes the intermittent control portion for controlling the intermittent signal, 61 denotes a zero-phase detector for detecting the instant the phase between the output signal of the prescaler 57 and the reference signal becomes zero, and 62 denotes an initial setting pulse generator for initializing the reference frequency divider 59 and the variable frequency divider 58.
Operation will be described below.
Generally, a frequency synthesizer includes a plurality of frequency dividers 58, 59, and when the loop is in a phase-locked state, the outputs of these frequency dividers 58, 59 are in synchronism at a certain phase difference. However, since the frequency dividers 58, 59 are each formed by a plurality of flip-flops, the relative phase between the outputs of the frequency dividers 58, 59 becomes unstable immediately after power is turned on again after the power has been turned off. Hence, a large error signal is outputted from the phase comparator 53 at this instant, whereby a frequency jump, that is, a momentary-great jump of the frequency of the VCO 56, occurs and makes signal reception impossible. To solve this problem, there have been devised various approaches both in software and hardware.
FIG. 2 shows the timing between a format of data received in a stand-by state and an intermittent signal controlling a frequency synthesizer in the U.S. 800 MHz band mobile radiotelephone system (EIA, IS - 19). According to standards in the system, in the standby state only A-words or only B-words may be read dependent on whether an even or odd telephone number has been assigned to the mobile telephone. Control words A and B are generated from a base station and instruct the controller to switch to a particular frequency dependent on various reception conditions. If the frequency jump period is not longer than 4.4 ms when power is turned on again by the intermittent signal being applied as shown in FIG. 2, there is no problem in receiving data. The design of hardware to satisfy the above described condition is indicated in FIG. 1. When power is turned on again, the instant the momentary phase between the output signal of the prescaler 57 and the reference signal becomes zero as detected by the zero-phase detector 61, the reference frequency divider 59 and variable frequency divider 58 are initialized by setting pulse generator 62, thereby preventing a large phase error signal. Meanwhile, the portion enclosed by one-dot chain line in FIG. 1, which is composed of a CMOS LSI, is continuously powered.
Since prior art mobile telephones are constructed as described above, a zero-phase detector 61 and a phase-locked loop with an intermittent operation controlling function have been used and the construction has therefore been complex and expensive. There have also been problems in that a frequency drift occurs when the phase-locked loop ceases operation and a large change in the frequency is caused specifically by vibration or the like. Further, since the signal receiving portion and the VCO, high-frequency amplifier, etc., of the frequency synthesizer are continuously powered, namely, operated without intermission, there has been a problem that power consumption has been excessively high.